Из http://en.wikipedia.org/wiki/NATO
Purpose
The core of NATO is Article V of the NATO Treaty, which states:
The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all. Consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.
This provision was intended so that if the USSR and its allies launched an attack against any of the NATO members, it would be treated as if it was an attack on all member states. This marked a significant change for the United States, which had traditionally favoured isolationist policies. However, the feared invasion of Western Europe never came. Instead, the provision was invoked for the first time in the treaty's history on September 12, 2001, in response to the September 11 attacks on the United States the day before.